CHAP. II.] 



A NERVOUS SYSTEM. 



31 



The ultimate bundles of elementary ' fibrils ' are 

 gradually aggregated into larger bundles, or 'fibres,' as 

 they recede from their seats of origin or termination and 

 approach the nerve centres with which they are in com- 

 munication. These smaller bundles soon become enve- 

 loped in a very delicate membranous sheath (Schwann), 

 whilst the component fibrils fuse more or less completely, 

 so that the fibre appears either 

 structureless (fig. 3), or merely 

 shows signs of fibrillation. A little 

 further on these still small fibres 

 become enveloped by a layer of 

 white semi-fluid ' medullary sub- 

 stance/ which lies beneath the 

 membranous sheath of Schwann, 

 and forms a white border to the 

 nerve as it is seen on micro- 

 scopical examination. Thus a 

 dark bordered, white, or medul- 

 lated nerve fibre is formed. 



Such dark-bordered fibres are 

 at first very slender ; but by co- 

 alescence with others of the same 

 kind larger fibres are produced 

 (fig. 4), varying in man from T2lToo tn to ^oVo^ of an 

 inch in diameter. The central portion of such a nerve 

 fibre, viz., that lying within the white medullary sheath, 

 is its most important constituent ; it is almost translucent, 

 and is known as the axis band or axis cylinder. In 

 the perfectly fresh state it shows faint traces of fibril- 

 lation, but unless examined with care it may appeal- 

 structureless, and yield no evidence to the microscopic 

 observer as to its compound nature. Under the influence 

 of slight traction, or by imbibition of water, these rnedul- 



Fio. 3. Human Nerve Fibres 

 of different sizes (KSlliker). 



a, a, a. Healthy fibres, the 

 largest of .which is ' dark-bor- 

 dered.' b, b. Fibres altered by 

 exposure. Magnified 350 dia- 

 meters. 



